Cryptology ePrint Archive: Report 2003/078

Relation among simulator-based and comparison-based definitions of semantic security

Yodai Watanabe and Junji Shikata

Abstract: This paper studies the relation among simulator-based and
comparison-based definitions of semantic security.
The definitions are considered in a more general framework
than the ordinal one; namely, an adversary is assumed to have
access to prior information of a plaintext.
If the framework is restricted to the ordinal one,
then all the security notions considered in this paper,
including indistinguishability, are shown to be equivalent.
On the other hand, the equivalence is not necessarily
valid in the general framework.
In fact, it is shown that no encryption scheme is secure
in the sense of comparison-based semantic security
in the strongest forms. Furthermore, a sufficient condition
for the equivalence between semantic security
and indistinguishability is derived.